"Steel" is a general term that refers to iron alloys having over 50% iron and up to about 1.5% carbon, as well as additional materials. There are a number of known steel compositions. For instance, certain iron-chromium alloys having from about 12% to about 18% chromium and about 8% nickel are referred to as stainless steels. Other materials, such as molybdenum, manganese and silicon, also are routinely added to iron alloys to provide desired characteristics. Certain materials may be added to molten steel compositions to effect deoxidation, control grain size, and to improve mechanical, thermal and corrosion properties. Iron alloys of different chemical compositions have been developed to meet the requirements for particular applications.
Steel compositions also can be processed to have various microstructures, including pearlite, bainite and martensite microstructures, by varying the composition and heat processing steps. Martensitic materials generally have a relatively high strength, but are not very ductile. Pearlitic materials have the reverse characteristics, that is relatively low strength but high ductility. When bainitic and martensitic materials have equivalent hardnesses, the bainitic materials typically are less strong than the martensitic materials, but also are more ductile. Thus, the bainitic materials exhibit a good combination of both strength and ductility.
Bainite microstructures typically are formed in an isothermal transformation process. To produce materials having a bainire microstructure, a steel composition is rapidly cooled from a fairly high temperature of greater than about 1500.degree. F. (the austenitizing temperature) to a temperature of about 475.degree.-650.degree. F. (the austempering temperature). The steel composition is austempered for a sufficient period of time to complete the transformation of the steel composition from an austenite face-centered cubic microstructure to a bainite body-centered cubic structure. The time and temperature required to produce different microstructures are interrelated.
Steel compositions have been used for years to make tools for working and forming metals, wood, plastics and other materials. These devices must withstand high specific loads, and often operate at elevated or rapidly changing temperatures. This creates problems, such as stress failure, when steels are in contact with abrasive types of work materials or subjected to shock or other adverse conditions. Ideally, tools operating at ambient conditions and under normal operating conditions should not suffer damage, unnecessary wear, or be susceptible to detrimental metallurgical changes.
Saw chain is one example of a device that is made from iron alloys. The iron alloys used to produce saw chain are chosen to balance several requirements, including, but not limited to, wear resistance, strength, fatigue resistance and toughness. These requirements have best been met for normal applications with an iron alloy that is substantially the same for all major manufacturers of saw chain. This alloy can be used for low-temperature applications, although the unique requirements for low-temperature applications indicate that a new alloy would be desirable.
Certain regions of the world routinely experience winter temperatures colder than 0.degree. F. As a result, certain jobs require using steel tools which perform satisfactorily at temperatures at least as low as 0.degree. F., and perhaps as low as about -50.degree. F. Steel devices operating under these conditions have particular operating requirements. Previous attempts to form steel compositions having enhanced low temperature toughness have generally proved to be unsatisfactory.
There are patented approaches to improving the toughness of steel alloys. Merkell et al.'s U.S. Pat. No. 3,854,363 (Merkell), which is incorporated herein by reference, discloses a steel composition that is particularly designed to have good wear resistance. However, Merkell also states that:
The remarkably good toughness of the chain saw unit according to the invention, compared to corresponding quality of conventionally made units, consisting of saw chains and guide plates, has been produced by carefully adjusted carbon content of the steel alloy in combination with the alloying elements Si, Cr and Mo and/or W. PA1 Merkell further states that: PA1 By making the links, for instance the cutter links, of the normally austempered steel according to the invention, i.e., the toughness is increased most essentially, not least at the cutting edge. As examples of preferably used steel compositions, identified in percentages by weight, may here be mentioned: PA1 0.6-0.7 percent carbon, 1.0-1.4 percent silicon, 0.30-0.45 percent manganese, 0.4-0.6 percent chromium, 0.2-0.4 percent molybdenum, 0.1-0.2 percent vanadium, and the remainder iron with a normal small amount of impurities.
Merkell, column 2, lines 28-34. Emphasis added. PA0 Merkell, at column 3, lines 26-36.
In summary, the prior art teaches that toughness can be enhanced by: (1) decreasing the carbon content of the alloy; (2) increasing the nickel content of the alloy [see, for instance, Alloying Elements in Steel, 2nd Ed., page 244, American Society for Metals (1961)]; or (3) increasing the silicon concentration in the alloy (Merkell). These options are unsatisfactory. Reducing the carbon content reduces both the strength and the wear resistance. Increasing either the nickel content or the silicon content significantly increases the cost of the alloy. Moreover, increasing the silicon content makes the alloy hard to process because such alloys tend to crack, particularly during hot rolling or continuous casting procedures.